Quantifying devitrification and porosity in thermally grown oxides through spatially-resolved time-domain thermoreflectance

Abstract

As silicon carbide (SiC)-based ceramic matrix composites replace metals in state-of-the-art gas turbine engines, new thermal and environmental barrier coating (EBC) systems must be designed to accommodate the change in turbine blade material. Ytterbium disilicates have been demonstrated to be a promising candidate for these EBCs, but further research is needed into their degradation mechanisms to improve their lifetimes during thermal cycling typical in engine operation. One point of failure is spallation due to crystallization of the thermally grown oxide (TGO) at the interface between the rare earth disilicate and the turbine blade. As the EBC system is thermally cycled in steam, water and air react with underlying Si containing coating and/or substrate layers, forming amorphous silica. Over many cycles, this TGO crystallizes to form cristobalite, which experiences significant volume strain as it is thermally cycled, resulting in the formation of pores and cracks at the interface of the TGO and silicate topcoat, eventually leading to the spallation of the coating from the turbine blade. In this work, we present a method for measuring cristobalite formation in EBCs by using time-domain thermoreflectance (TDTR) to map spatial variations in thermal conductivity with micrometer resolution. We then quantify the devitrification of the TGO layer based on the distribution of thermal conductivity measured throughout the TGO and measure the resulting degree of porosity at the interface as a gauge of coating effectiveness and delamination toughness.